Electric discharge device



NOV l, 1938. E, A. MASSA, v.JR 2,135,039

` ELECTRIC DISCHARGE DEVICE Filed March 51, 193e fw fw @d Patented Nov. 1., 1938 OFFICE:

iiLECTRIef nisoHAn'Gm DEVICE Ernest A. Massa, Haddon Heights,v N. J., yas-k sgnor to .Radio Corpor poration of .Delaware ationof America, a co1'.-

Application March 321., .1936, Serial' No. 71,8755

This invention relates to electric discharge devices and particularly to devices of the type wherein amplification of a primary electron current is obtained through utilization of secondary `-emis- More specicallygthe invention relates to improvements in electron multipliers of thegeneral type disclosed in the -copending application of' Ernest A. Massa et al., Serial No. 33,996,1iled 4July 31, 1935, and assigned toRadio Corporation of America.` A

In the .Massa application, there is `disclosed a multi-stage electron multiplier 'constituted by a plurality of pairs of -opposedelectrodes axially vdisposed in an. evacuated containerwith vtheir sensitization, to enhance their abilityv toj emit' secondary electrons, and aY primary'electronsource, photoelectric or therminic in character,

is provided. In addition, electromagnetic means are vprovided for the purpose of causing the elec-.N trons to describe approximately trochoidal paths from each lower electrode to the next adjacent electrode, thus permitting the use of high ac-` celerating potentials on the upper electrodes Withiout the disadvantages resulting'vfrom the space chargesericountered in a device constructed according to the prior art. The ratio of the number of secondary electrons to the number of primaryelectrons depends, in

glpart, upon the character of thesurface and upon the potential difference. between the surfaceand the source ofthe electrons that impinge upon it. This ratio can be made considerably greater than unity. For, example, aA ratio of iive or more secondary electrons to one impinging electron-is readilyobtainable with metallic surfaces treated in knownways and .subjected to discharges at potentials of 200 to 400 volts. y Y l.

If the secondary electron current, in turn, is

caused to impinge with suiiicient velocity upon a amplification of the original or primary modulated or unmodulated velectron vcurrent equivalent to theampliiication per electrode raisedto the nthY power. i

. Afmlt'i-stage electron multiplier; is capable of. Y

1o claims--xl (c1. 25o-27.5)

givingan` amplification of several millionltimes the` primary'- current. Wherel such relatively enormousgains'are achieved in a single envelope vthere is ra pronouncedtendency for the device to breakinto oscillation andVv to l generate spurious and `undesired frequencies -of oscillation. This phenomenon-is thought to obtain by reason of the l inherent internalfcoupling existing between theV collecting plate and the several multiplying electrode elements but may be the result of other less apparent causes-peculiar to such high gain de- V1CeS;` Y

Itis, accordingly, an object'of the invention'to obviate the generation of lso-called "feed-back oscillationsA in devicesof the type whereinampli` iica'tionfofa `primary electric currentis obtained through utilizationfof the phenomenon of secondary `emission.'

` Another object'of the invention isv to provide an electron multiplienj the operation ofgwhich is characterized byr substantial freedomfrom distortion:

The `above andI otherl objects are accomplished in .accordance with the invention by' the provision of an auxiliary electrode' in the form of a grid positioned'in the path of the secondary electrons and preferably directly connected to the accelerating electrodenearest the collecting'plate or anode of the device.V The grid-is preferably of specialdesig'n and-is sopositioned Withrespect to the anode as to present a substantially negligible barrier tothe passage of the electrons.

The Vnovelgfeatures characteristic of the inventionv are Y setd forth with particularity, .in the appended claims; The invention itself,vhowever, both as to its organization and its method of operation; togetherwith additionalobjects and adi/'.antages,V will bevbest understood from the following description of `certain specific embodiments thereof, lwhenread in connection with lthe ac,-`

companying drawing, in whichk Figure 1 is a diagrammatic representation of a multi-electrode :photo-,sensitive electron multiplier embodyingv the invention,

Fig. 2 is an' end-view and Fig. 3 is a side elevationbfi an electrode assembly including one type ofgrid,

. Fig. 4 isfan end View and Fig. 5 is a side elevation of an yelectrode assembly including a grid `of preferred alternate construction,

Fig. 6 is a .plan view ofthe patternformed upon the output'electrode by thelelectrons which have Y passedthroughithe grid of Figs.4 and 5, and I Fig. '7 is'a.Y chart of the plate characteristicsVv orf.

a ten-stage multiplier including a grid of the type illustrated in Figs. 4 and 5.

An electron multiplier constructed in accordance with the principle of the invention is diagrammatically illustrated in Fig. 1. Light entering the tube 5 and striking a photo-sensitive electrode l0 releases primary electrons which are drawn upward by an electrostatic eld, provided by the upper or accelerating electrode 20 but are deected by a constant magnetic eld (from a source not shown) perpendicular to the plane of the paper. When the voltages and eld are properly adjusted the adjacent lower or multiplying electrode Il is struckV by the group or bundle of primary electrons, emanating from the primary emitter I0, and secondary electrons are emitted. The secondary electrons from the point of impact encounter electrical conditions sub-4V stantially identical to those met by the primary trate and todirect the electrons in the manner electrons and become increasingly more numerous as they impinge successively upon the. multiplying electrodes I2-I5. After several Vsuch steps the output is taken from the target or anode 45 at the opposite end of the tube.

Back-.coupling'and the generation of YVfeedback oscillations is prevented, in accordance with the invention, by the provision of an auxiliary electrode 35 which may be directly or indirectly connected to the accelerating electrode 25 vnearest the output electrode 45. This auxiliary electrode 35 is of grid-like construction and eX- tends downward towards, but not touching, the last multiplying electrode l5 in a plane normal thereto'and directly across the path of the multiplied electron stream.

While various types of grid construction will prevent the tube from oscillating, it has been found that when it is of ordinary meshed'construction the output current willusually be distorted. Thus, it was found that a 30 mesh screen built of .006I nickel wire inclu-ded in a ten-stage multiplier tube connected to a load of '70,000` ohms prevented the tube from oscillating, but harmonic distortion was found to be substantiallyV '28% at 0.28 watt output. The per cent distortion was discovered to be a function of the quantity of electronA current absorbed by the screen.

timum performance is-achieved when the wire area is approximately 2% of the cross sectional area of the electron beam.

Figs. 2 and 3 show a grid constructed and poy sitioned in the above described preferred manner. In these figures the grid is constituted by afframe 35a, suspended from the upper accelerating electrode 25, and having a plurality of spaced parallel wires 35h, 35e, etc. spanning the path of the electrons in a vertical direction. These wires 35h-35o, etc. are spaced one from another a distance substantially equal to the distance from the plane of the grid and the plane of the output electrode 45 .(Fig. 3) In a standard tube this distance may `ordinarily be of the' orderof 11g of an inch.

The grid 35 of Figs. 4 and 5 is constituted by a continuous wire 351.1:l which crosses and recrosses the path of the electrons in a horizontal direction. Where the tube is provided with a pair of spaced insulating strips M-M of mica or the like (as in application Serial No. 33,996) for supporting the paired electrodes I5' and 25', these strips are provided on each side with a pair of orifices n through which the wire 351.0 is threaded. One terminal of this wire is connected as indicated at 35t, to the upper or accelerating electrode 25' and the other end conveniently bent over the micakstrip M as indicated at 35s. The sections of the grid wire 35m like the spaced wires 35h-350 of Figs. 2 and' 3 are spaced one from another a distance substantiallyequal to the distance from the plane of the grid and the plane Vof the output electrode 45.v

` In operation, the grid vwires appear to concenand formed ofV .00 1 tantalum wire spaced 11e" from the collector, areshown inthe Ychart of Fig. '7. The plate current, measured in milliamperes at different values (-1,72 and 4) of light intensity, is .represented along the `ordinate of the chart and plate voltage, with respect to screen or grid voltage, is represented along the abscissa of the chart. Line A-C represents the load on the output of the tube (in ohms) which determines the amount of R. M. S. power obtained from the tube. Point BV on line A-C coincides with a `It is bepoint midway 'along the ordinate between zero kand maximum light intensity. Zero distortion would be achieved if B were also the midpoint of AC, but this ideal condition can only obtain where the curves of light intensity extend straight to the zero ordinate, as would be the case if the screen collected absolutely no current when the plate voltage approaches the screen voltage.

From the foregoing it is apparent that the percentage of harmonic distortion may be math matically expressed by the formula AB Y and applying this formula in the instant case a quantitative value of 2.9% at .27 watt (output) is obtained. Y L

The various embodiments of the invention Y which have been illustrated for the purpose of tas:

explaining the inventive concept are susceptible of various modic'ationsvwhich will be apparen Yto those skilled in the art. Y

What is claimed is:

1. An velectron tube comprising, an electrode surface adapted to liberate electrons by secondary emission, a collector electrode, an accelerating electrode for directing electrons'from said surface to said collector electrode, and a control` surface to said collector electrode and a control electrode of grid-like construction connected to said accelerating electrode and positioned in the path of said electrons and presenting a substantially negligible barrier to the passage of said electrons.

3. The invention as set forth in claim 2 Wherein the area presented to the electrons of the Wires constituting said grid-like electrode is less than substantially 5% of the cross-sectional area of the path of said electrons.

4. The invention as set forth in claim 2 Wherein the area presented to the electrons of the Wires constituting said grid-like electrodeis of the order of substantially 2% of the cross-sectional area of the path of lsaid electrons.

5. An electron tube comprising, an electrode surface adapted to liberate electrons by secondary emission, a collector electrode; an accelerating electrode for directing electrons from said surface to said collector electrode, and a grid-like member connected to said accelerating electrode and positioned in the path of said electrons, the Wires constituting said grid-like member being spaced one from another a distance substantially equal to the distance between the plane of said wires and the plane of said terminal electrode.

6. 'I'he invention as set forth in claim 5 Wherein said grid-like electrode is constituted by a metallic frame and a plurality of Wires each parallel with the others and joined to said frame.

i terminal electrode positioned to receive electrons from said emissive surface, and a grid-like member inthe path of said electrons and supported by said strips and connected to said accelerating electrode.

9. The invention as set forth in claim 8 Wherein said strips are provided with a plurality of oriices and said grid-like member is constituted by. a continuous Wire passing through said orices and crossing and recrossing the path of said electrons.

10. An electron multiplier comprising a' plurality of secondary-electron emitters, means for directing electrons from one to another of said A emitters, an output electrode interposed in the path of .the secondary-electrons, and a foraminous electrode mounted closely adjacent said output electrode for subjecting the secondary-electrons to an oscillation suppressing potential.

ERNEST A. MASSA, JR. 

